Rat sciatic nerve fibers were demyelinated by injection of lysolecithin and examined at several stages as Schwann cells proliferated, adhered, and initiated remyelination. Immunoperoxidase electron microscopy was used to follow the clustering of Na+ channels that represents an early step in the formation of new nodes of Ranvier. Thin sections were examined at 80 kV using conventional transmission EM and thick sections (1-2 urn) were examined at 400 kV using the intermediate high voltage electron microscopy at NCMIR. IVEM was needed to provide nearly complete views of axons at high resolution in a single section. At the peak of demyelination, 1 week post-injection, only isolated sites suggestive of the original nodes, were labeled. As Schwann cells bound and extended processes along the axons, clusters of axonal Na+ channels were often found just beyond their leading edges. These channel aggregates were associated only with the axolemma and Na+ channels were not detected on glial membranes. Sites with more than one cluster in close proximity and broadly labeled aggregates between Schwann cells suggested that new nodes of Ranvier formed as neighboring Na+ channel groups merged. Schwann cells thus seem to play a major role in ion channel distributions in the axolemma. In all of these stages Na+ channel clusters were found primarily just outside the region of close contact between axon and Schwann cell. This suggests that either Schwann cell adherence acts to exclude Na+ channels, or that diffusible substances are involved and can act some distance from regions of direct contact. This work is to be published in the Journal of Neurocytology (Novakovic et al., 1996).